Cable Installation for Security Systems: Screened vs Unscreened, Segregation from Power and Cable Mapping

Quick Answer: Security system cabling must be segregated from mains power by at least 50 mm (100 mm at crossings) to prevent interference. Screened cable (with drain wire earthed at one end only) is required wherever electromagnetic interference is a risk — near motors, fluorescent lighting, or power cables. All cables must be concealed within secure areas where possible, tamper-monitored where exposed, and documented with an as-fitted cable schedule. BS 7671:2018+A2:2022 governs low-voltage segregation requirements.

Summary

Cable installation is the invisible infrastructure of every security system. Done well, it is permanent, reliable, and unobtrusive. Done poorly, it causes interference, false alarms, failed connections, and requires ripping out walls years later when problems emerge.

The security industry has specific cable requirements that differ from general electrical wiring. Security cables carry low-voltage signals (6–24 V DC typically) and are susceptible to electromagnetic interference from mains power cables, fluorescent lighting ballasts, motors, and RF sources. Screening, segregation, and correct earthing are not optional extras — they are the difference between a system that works flawlessly and one that generates constant false alarms or fails intermittently.

Documentation is equally important. A security system cable schedule is an asset — it enables fault finding, system expansion, and maintenance by anyone (not just the original installer). An undocumented system is a liability.

Key Facts

Quick Reference Table

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Cable Type Typical Use Cores × Area Screening Max Run
Alarm cable 6-core General detection zones 6 × 0.22 mm² Unscreened 100 m typical
PIR detector cable PIR detectors, vibration detectors 4 × 0.22 mm² screened Foil + drain 100 m
RS-485 data Keypad, expander, reader bus 1 × 0.22 mm² screened pair Foil + drain 1,200 m
CCTV RG59 coax Analogue cameras 75 Ω Braided 300 m
CCTV Cat6 IP cameras, PoE 4 × UTP pairs Unscreened (Cat6) / Screened (Cat6A STP) 90 m (PoE)
Door entry 2-wire Analogue intercom BUS 2 × 0.75 mm² Unscreened 200–300 m
Maglock cable Magnetic lock power 2 × 1.0 mm² Unscreened 50 m (size for current)
Stranded hook-up Panel internal wiring Various N/A N/A

Detailed Guidance

Screened vs Unscreened: When to Use Each

Unscreened cable is adequate for:

Screened cable is required for:

Braided screen vs foil screen:

Earthing the screen correctly:

Segregation from Mains Power

BS 7671:2018+A2:2022 Section 528 specifies minimum segregation distances. For security systems carrying SELV (Safety Extra Low Voltage — typically 12–24 V DC):

Condition Minimum Separation
Open routing alongside mains 50 mm
Crossing mains at right angles 100 mm gap or insulated crossing
In same conduit/trunking as mains NOT PERMITTED (unless Category 3 separation)
Separated by earthed metallic partition 0 mm (partition provides Category 3 separation)
In separate compartment of multi-compartment trunking 0 mm (compartment provides Category 2 separation)

Practical routing:

Voltage Drop Calculation

For DC-powered devices (PIR detectors, panels, locks), voltage drop on the cable is critical. A PIR at the end of a long cable run may not receive enough voltage to operate reliably if the cable is too light.

Formula:

Voltage drop (V) = (2 × cable length in metres × current in amps) ÷ (cable CSA in mm² × 56)

(56 = conductivity of copper in m/Ω·mm²)

Example: 100 m run, PIR drawing 15 mA, 6-core 0.22 mm² cable:

V = (2 × 100 × 0.015) ÷ (0.22 × 56) = 3 ÷ 12.32 = 0.24 V drop

On a 12 V supply, 0.24 V drop is 2% — acceptable. But a maglock drawing 0.5 A on the same 100 m cable:

V = (2 × 100 × 0.5) ÷ (0.22 × 56) = 100 ÷ 12.32 = 8.1 V drop

Unacceptable — the maglock receives only 3.9 V. Upsize to 1.0 mm² and the drop becomes 1.8 V (acceptable on a 12 V supply).

Cable Mapping and Documentation

Every professionally installed security system requires a cable schedule. This document is an asset that facilitates:

Minimum cable schedule content:

Additional documentation:

For NSI/SSAIB approved installations, as-fitted drawings are a compliance requirement and must be provided to the customer at handover.

Common Installation Faults and Prevention

Intermittent fault — cause: poor termination Use appropriate ferrules (bootlace crimps) on stranded conductors before inserting into screw terminal blocks. Bare stranded wire in screw terminals loosens under vibration over time. For PIR detectors, use 0.22 mm² ferrules; for power cables, use the correctly sized ferrule.

False alarms — cause: unscreened PIR cable in EMI environment Retrofit screening: separate the PIR cable from EMI sources, or replace with screened cable. As a temporary fix, add a 100 nF ceramic capacitor from each conductor to the signal earth at the PIR (damper capacitors against RF pickup) — but proper screened cable replacement is the correct fix.

Short circuit — cause: cable damage during installation Always draw a cable (pull the figure 8 slack out) before cutting to length. Use a cable puller for long runs in conduit rather than brute force. Never pinch cable in door frames, under skirting, or at penetrations. Use appropriate grommet/bushes at all penetrations.

Intermittent connection — cause: cable movement stress Dress cables with adequate loop at each device to allow for movement. Secure all cable runs at 300–400 mm intervals. At devices that may be removed for maintenance (PIRs, keypads), leave 150–200 mm service loop.

Frequently Asked Questions

Does security cable need to be red like fire alarm cable?

No mandatory colour requirement applies to intruder alarm cabling in UK standards. However, industry convention uses white or grey for security cables and red for fire alarm cables. Using colour consistently within your installations makes identification easier. Fire alarm cables to BS 5839-1 are a different matter — cabling for fire alarm systems has specific requirements for cable specification and route protection.

Can I run CCTV and alarm cables in the same conduit?

Yes, provided both are SELV (≤50 V AC or ≤120 V ripple-free DC) and there is no mains power in the conduit. CCTV coax and alarm signal cables are both SELV. However, if the CCTV cables carry 12 V or 24 V DC power (PoC — power over coax), keep them away from sensitive alarm signal cables due to potential interference.

What type of conduit should I use for external runs?

External cable runs should use UV-resistant HDPE or PVC conduit (minimum 20 mm diameter for small cables). Metal conduit provides better tamper resistance but corrodes unless stainless steel or galvanised and sealed at penetrations. Where conduit is buried, use HDPE duct rated for direct burial. All conduit entries to the building must be sealed against moisture ingress.

How should I label cables?

Use heat-shrink labels or adhesive labels rated for the temperature range (typically -30°C to +80°C). Label at each end of the cable and at each junction box or accessible point. Label format should match the cable schedule: cable reference, origin, and destination. Avoid labels that rely on handwriting — thermal printed labels are far more legible after years in a roof void.

What is LSZH cable and when is it required?

LSZH (Low Smoke Zero Halogen) cable is specified where, in a fire, toxic halogen gases from burning cable insulation pose a risk to evacuees. Standard PVC cable releases hydrogen chloride on burning — corrosive and toxic in enclosed spaces. LSZH is mandatory for fire alarm systems (BS 5839-1 requires FP or PH fire-performance cable on circuits requiring cable integrity). It is good practice for security cables in occupied buildings, hospitals, schools, and transport infrastructure. Confirm with the building's fire risk assessment and fire alarm designer.

Regulations & Standards